Terracotta Greenware developing dark marks when drying

Hi. Can anyone advise why a red terracotta clay would develop a dark line as it dries. These marks do not fire out. I have used the same clay and never had this problem but one of my students who works at home is having this issue. If this was fabric and not clay, I would describe it as a dirty water stain with a dark edge. As I don't see what the student is doing at home, I cannot put my finger on the problem, but having questioned her, she does not seem to be doing anything wrong. Any comments much appreciated.

Thanks for the responses - I somehow didn't pick them up at the time.. battling with this site.I think I found the answer to my ugly black mark problem on htp://www.claymonk.com/blog/2011/01/getting-rid-of-the-jet-black-area/ There are pictures of the undesirable black as well as the more desirable effect. It appears it is the proximity of the fuel to the work but I also think it was the type of fuel I used. Someone brought me loads of bark strips and I think they are giving that shiny black gum look. So will try again without them when our weather here cools down. Too dangerous now for fires. Pit and pots all ready. Cant wait!